1. Field of the Invention
This invention relates to transformers and, more particularly, is directed to a transformer having particular utility with switching regulators.
2. Description of the Prior Art
Conventionally, transformers which are used with switching regulators include a tertiary winding structure, in addition to conventional primary and secondary winding structures. Such tertiary winding structure may be used, for example, for controlling a pulse width modulation (PWM) circuit which controls the power supplied to the primary winding structure. In other words, the voltage across the tertiary winding structure, which is proportional to the voltages across the primary and secondary winding structures, is rectified and used for controlling the operation of the primary winding structure.
However, because of the manner in which the primary, secondary and tertiary winding structures have conventionally been wound about a bobbin secured to a magnetic core of the transformer, there exists an unsatisfactory degree of magnetic coupling between the tertiary winding structure and the primary and secondary winding structures so that the detected voltage from the tertiary winding structure may not reflect, that is, may not be proportional to, the voltages at the primary and second winding structures. For example, in the case where the primary, secondary and tertiary winding structures are comprised of first and second primary, secondary and tertiary winding portions, respectively, it has been known to wind the primary, secondary and tertiary winding structures on the bobbin in the following order: the first primary winding portion, the first tertiary winding portion, the first secondary winding portion, the second secondary winding portion, the second tertiary winding portion, and the second primary winding portion. With such arrangement, the first tertiary winding portion is sandwiched between the first primary and secondary winding portions so as to provide a high degree of magnetic coupling between the first tertiary winding portion and the first primary and secondary winding portions. However, the degree of magnetic coupling between the first tertiary winding portion and the second primary and secondary winding portions is poor. As a result, the transformer may have an unstable characteristic associated therewith and may, in particular circumstances, even produce an undesirable oscillation state. It should therefore be appreciated that, since the magnetic coupling between the first tertiary winding portion and the second primary and secondary winding portions is not high, the tertiary winding structure may not accurately detect a voltage proportional to that of the primary and secondary winding structures.